Chlorine analysis has wide commercial applicability, and is useful for testing drinking water, pool and spa water, aquarium water, industrial and environmental water, and for other types of water testing. Medical applicability includes testing of equipment such as dialysis equipment to confirm removal of chlorine and chloramine contaminants. Chlorine can be present in water as free available chlorine and as combined available chlorine. Both forms can be determined together as total available chlorine. Combined available chlorine exists as monochloramine, dichloramine and other chloro derivatives. If monochloramine is present in a sample, its interference with free chlorine analysis is dependent on sample temperature, relative concentration of monochloramine to free chlorine, and the time required to perform the analysis.
Quantitative methods for chlorine analysis that rely upon the evaluation of sample color are described in Standard Methods for the Examination of Water and Wastewater, 19th Edition, 1995 (4500-Cl), and include a DPD calorimetric method. According to the DPD colorimetric method (4500-Cl G) for free chlorine, 0.5 ml of a phosphate buffer solution and 0.5 ml of a DPD indicator solution are added to a photometer cell, 10 ml of a sample is added to the photometer cell and mixed to provide the sample with an appropriate pH and promote the color reaction, and the sample color is read immediately using appropriate photometric equipment. An immediate reading is desirable for free chlorine because of monochloramine interference and color complex degradation, which are reduced or avoided by taking the reading within one minute, especially within 30 to 45 seconds, of combining the sample and the analytical agents. As can be deduced, the less time to perform a free chlorine analysis, the more accurate the analysis should be.
The DPD indicator solution is prepared by dissolving an appropriate amount of DPD oxalate, DPD sulfate pentahydrate or anhydrous DPD sulfate in water containing sulfuric acid and disodium EDTA. In the case of the anhydrous sulfate, 0.5 ml of the DPD indicator solution contains 0.00055 g of the DPD salt.
For accurate results, pH control is essential to minimize interferences. Too low a pH may enhance monochloramine interference with free chlorine analysis. According to this US EPA accepted, DPD colorimetric standard method, the phosphate buffer solution is prepared using an about 2:1 weight ratio of potassium phosphate monobasic anhydrous to sodium phosphate dibasic anhydrous, with disodium EDTA, in an amount sufficient to provide a typical 10 ml sample with a pH in the range of about 6.2 to 6.5. The EDTA additive may advantageously overcome one or more test interferences.
For total chlorine, about 0.1 g of potassium iodide may after the free chlorine analysis, be added to the sample, and after 2 minutes, the sample color is photometrically analyzed. US EPA accepted method 330.5 for total chlorine in natural and treated waters, corresponds to the foregoing total chlorine analysis except that potassium iodide is added with the DPD indicator and phosphate buffer solutions. Method 330.5 specifies the use of DPD oxalate or sulfate, and measurement after 2 minutes.
The photometric equipment is appropriately calibrated in advance. A spectrophotometer for use at a wavelength of 515 nm and providing a 1 cm or longer light path, or a filter photometer with a wavelength measurement range of 490 to 530 nm, and a 1 cm or longer light path may be used.
In the photometric analysis art, after a BLANK reading, a photometer cell is often removed from a photometer for the addition of the analytical agents to the cell, and thereafter is re-inserted in the photometer for the actual reading. To minimize variability of measurements, a user is therefore instructed to re-insert the photometer cell so that the cell has the same orientation. Otherwise, variability in the geometry and quality of the cell glass can cause variability of measurements. Furthermore, the exterior of a photometric cell should be free of smudges or fingerprints or a water drop to ensure an accurate reading.
Commercially available, prior art technology that is based upon DPD colorimetric methodology, includes powder “pillows” (carboxylate salt, DPD salt, and sodium phosphate dibasic mixed together), and evacuated reagent ampoules with frangible tips. This prior art technology provides the analytical agents as a mixture, and specifies photometric analysis of free chlorine within one minute after combining the analytical agents and sample. For total chlorine analysis, KI reagent is also used.
Another prior art colorimetric technology for water testing in which DPD is used for chlorine analysis, is described in U.S. Pat. No. 6,004,820 to Brayton. That patented technology is based upon a closed system. As explained by Brayton, a problem with powder “pillows” is that while a “pillow” contains the precise amount of reagent needed, reagent after opening the “pillow”, can be a safety hazard through hand contact or dust inhalation, or can be partially spilled with resultant possible test error.
The commercially available, prior art evacuated glass ampoules contain a pre-measured mixture of the analytical agents. In use, the ampoule tip is broken off. Contact with a broken ampoule tip can result in injury.
Other commercially available, prior art colorimetric technology is based upon crushable DPD tablets. Certain brands of these tablets include a mixture of a DPD salt and boric acid. A problem with DPD tablets, as well as with DPD powder, is that not all solids may dissolve within the specified allowed time. Moreover, incomplete solids dissolution can be expected to be worse when a sample is cold, with any cloudiness from undissolved solids potentially enhanced in the winter. As indicated, if additional time for complete solids dissolution is allowed, color complex degradation and/or monochloramine interference may result in loss of precision. Undissolved solids can lead not only to a concern about the accuracy of test results but also to lack of reproducibility. Also, DPD tablets need to be crushed, and crushing the tablets introduces an extra and time-consuming step.
As illustrated by U.S. Pat. No. 3,937,613 to Rosicky and U.S. Pat. No. 4,275,031 to Fisher et al, prior art reagent delivery devices that include a support such as an inert plastic strip or the like, and that release analytical agents for calorimetric analysis of chlorine are known. The sufficiently rigid support of such devices may be used to stir the sample, and thereafter the sample color is calorimetrically analyzed.
Rosicky teaches away from the use of DPD for chlorine analysis, for reasons of toxicity and allergenicity, and describes the combination of p-ethyl-oxyethylamino-aniline (EOAA) and non-ionic high molecular weight propyleneglycol ethyoxylates (PGE 8300 in particular) as an impregnant for filter paper fixed to a support, or as a coating on a support. The ethoxylates are described as a dispersing agent and protectant against oxidation, and as non-reactive with chlorine. Rosicky teaches a 20-40% w/v loading of PGE 8300 in an impregnating or coating solution, and visual comparison of the sample color with a color scale.
For chlorine analysis, Fisher et al (Example 6) teach the use of polyvinyl alcohol as a water soluble embedding polymer for DPD sulfate, and a waiting time of 10 minutes, instead of an immediate reading. The polyvinyl alcohol embedding polymer dissolves in the sample and releases the DPD sulfate. Thus, it appears that DPD release is retarded by the embedding polymer. In Example 6, Fischer et al disclose photometric analysis of the sample color, but do not disclose a phosphate buffer system.
Despite the foregoing advances, there continues to be a need for touch-free chlorine analysis of water that will advantageously minimize manipulations and reduce variability of measurements. Beneficially, the touch-free chlorine analysis will provide quick free chlorine analysis, thereby reducing monochloramine interference with free chorine analysis. Advantageously, the touch free-chlorine analysis will be based upon an accepted standard method for compliance testing, and in particular provide for immediate spectrophotometric analysis of free chlorine. Advantageously, there will no longer be concerns about powder spillage, or about whether adequate solids dissolution has taken place or whether undissolved solids from incomplete solids dissolution may interfere.